Bipolar transistor

ABSTRACT

A bipolar transistor using a B-doped Si and Ge alloy for a base in which a Ge content in an emitter-base depletion region and in a base-collector depletion region is greater than a Ge content in a base layer. Diffusion of B from the base layer can be suppressed by making the Ge content in the emitter-base depletion region and in a base-collector depletion region on both sides of the base layer greater than the Ge content in the base layer since the diffusion coefficient of B in the SiGe layer is lowered as the Ge contents increases.

This is a continuation application of U.S. Ser. No. 09/376,352 filedAug. 18, 1999, now U.S. Pat. No. 6,388,307.

FIELD OF THE INVENTION

The present invention concerns a bipolar transistor having an SiGe alloybase layer and, more in particular, it relates to a bipolar transistorsuitable to a ultra-high-speed digital IC and microwave or millimeterwave wireless communication IC, as well as a communication system inwhich the bipolar transistor is applied.

BACKGROUND OF THE INVENTION

Prior art related to a bipolar transistor using B-doped SiGe alloy (SiGealloy) for a base is described in Technical Digest of InternationalElectron Devices Meeting (IEDM), 1997 (pages 791-794). The SiGe basebipolar transistor of the prior art is to be explained with reference toFIG. 17(a) and FIG. 17(b). Graphs in the drawing show typical twoexamples of a depth profile of impurity concentrations and Ge contentsin an active region of a SiGe base bipolar transistor of the prior art(refer to FIG. 4 showing a cross section for the main portion of atransistor; along broken line A) of the prior art. In the graphs, aregion of a shallow depth is for an emitter and a portion with a largedepth is for a collector. In the example shown in FIG. 17(a), the Gecontent increases at a constant grade toward the collector in a regionof a base layer in which B is doped, reaches a maximum value at an edgeof the base layer, and is kept at the maximum value in a region of apredetermined width in a base-collector depletion layer. In the exampleshown by FIG. 17(b), the Ge content has a constant value in a region ofthe base layer in which B is doped and in a region of a predeterminedwidth in the base-collector depletion layer.

For improving the operation speed of a bipolar transistor, it isnecessary to shorten a base transit time of carriers and lower a baseresistance. For this purpose, it is necessary for the depth profile of Bin the base layer to make the width as narrow as possible and increasethe concentration as high as possible. However, in the prior artdescribed above, since B in the base layer is diffused by a heattreatment after forming the base layer, it results in a problem that thedepth profile becomes broad to widen the width. In addition, since thebroadening for the width of the profile due to diffusion increases asthe concentration of B is higher, it was difficult to make therestriction for the width of profile and increase of the concentrationcompatible with each other.

Another problem in the prior art transistor is that an effective basewidth is widened to lower the operation speed if the collector currentis increased. This is caused by injection of holes for neutralizing thenegative charges due to increase of a collector current density in aregion at low impurity concentration, so that an energy band structurefor a base-collector junction is changed as shown in FIG. 18.

FIG. 15 shows dependence of a diffusion coefficient of B in an SiGealloy on a Ge content. As the Ge content increases, the B diffusioncoefficient decreases to lower the diffusion speed. The problem ofincreasing the depth of B can be solved by utilizing this phenomena byadopting the following means.

If the Ge content is increased in a region adjacent to the B-doped layerof the base, that is, in an emitter-base depletion region or abase-collector depletion region, since the diffusion speed of B in theportion is lowered, widening of the width in the depth profile of B canbe suppressed.

When taking notice only on the decrease of B diffusion, the total Gecontents may be increased in the emitter-base junction depletion region,the base layer, and the base-collector depletion region, but the totalamount of Ge in the SiGe alloy layer is excessively increased.Therefore, this causes strong stresses in the SiGe alloy layer due tothe difference of covalent bonding radius between Si and Ge, to cause aside-effect of forming crystal defects.

The side effect can be suppressed by controlling the Ge content higherin a portion adjacent to the B-doped layer of the base and lower in theB-doped layer as much as possible. Since the total Ge content can bedecreased by this structure, occurrence of crystal defects can besuppressed. In this case, broadening of the distribution of B caused bythe reduction of the Ge content in the B-doped layer is almostnegligible. This is because the distribution of B is generally uniformin the B-doped layer, so that B is less diffused and the degree of thediffusion coefficient in this region gives less effect on the broadeningof the depth profile of B. For lowering the Ge content in the B-dopedlayer and kept it high in the portion adjacent therewith therebydecreasing the total Ge content as low as possible, the Ge concentrationmay be changed abruptly as much as possible at both ends of the B dopedlayer.

Further, the effect of suppressing the broadening of the B profilecompared with the prior art can be attained also by making the Gecontent in the emitter-base junction at least equal with the content inthe B-doped layer though it is not higher than the content in theB-doped layer, by the same reason as described above.

Further, the problem that the effective base width is increased by theincrease of the collector current can also be improved by the methoddescribed above of changing the Ge concentration as sharp as possible atthe edge of the B-doped layer on the side of the collector. This reasonis to be explained below. In a transistor of the prior art, an effectivebase width is increased as the collector current density increases asshown in FIG. 18 since the grade of the band become less steep at theedge of the base on the side of the collector. On the other hand, whenthe Ge concentration is changed abruptly at the edge of the B-dopedlayer, a nodge is formed in a valance band at a position at which the Geconcentration changes as shown in FIG. 2. The reverse V-shaped nodge onthe side of the collector has a function of hindering the effect ofsmoothing the grade of the band in that portion when the collectorcurrent density increases. This is because a great amount of holes areaccumulated at that portion if the grade is reduced and electricalneutral can no more be kept in the vicinity thereof. The effect ofsuppressing the change of the gradient of the band with increase of thecollector current density can improve the problem of lowering theoperation speed in a case of increasing the collector current.

SUMMARY OF THE INVENTION

Based on the considerations as described above, the present inventorprovides a bipolar transistor of the following structure and acommunication system by applying such a bipolar transistor.

At first, a bipolar transistor using a B-doped Si and Ge alloy (SiGe)according to the present inventor has a basic feature in that themaximum value of Ge content in an emitter-base junction depletion regionand a base-collector junction depletion region is greater than anaverage value in the base layer. In this bipolar transistor, it ispreferred that the grade of Ge content in a region in which the Gecontent is increased from the vicinity of the edge of the base on theside of a collector to the collector is made greater than the averagegrade of Ge content in the base layer.

Further, another feature of the present invention resides in a bipolartransistor using a B-doped SiGe alloy (SiGe) for a base in which themaximum value of the Ge content in the base-collector junction depletionregion is set greater than the average value in the base layer, whereinthe grade of Ge content in the region in which the Ge content isincreased from the vicinity of the edge of the base on the side of acollector to the collector is made greater than the average grade of theGe content in the base layer.

In a further aspect of a bipolar transistor according to the presentinvention, a bipolar transistor using a B-doped Si and Ge alloy (SiGe)for a base in which the maximum value for the Ge content in thebase-collector depletion region is set greater than an average value inthe base layer has a region in which the Ge content is constant from theedge of the base layer on the side of the emitter to the emitter-basejunction. In this structure, it is preferred that the grade of Gecontent in the region in which the content increases from the vicinityof the edge of the base layer to the collector is made greater than theaverage grade of Ge content in the base layer.

Further, in an optical transmission system according to the presentinvention comprising

an optical receiver system having an a photodetector for receiving anoptical signal and outputting an electric signal, a first amplifier forreceiving the electric signal from the photodetector, a second amplifierfor receiving the output from the first amplifier, a decision circuitfor converting the output from the second amplifier into a digitalsignal in synchronization with a predetermined clock signal, and acircuit for separating and converging the digital signal, and

an optical transmitter system having a circuit for synthesizing multipledigital signals, a semiconductor laser and a semiconductor laser driverfor driving the laser,

at least one of transistors in the first and the second amplifiers, thedecision circuit, a multiplexer for digital signals, a multiplexer formultiple digital signals and the semiconductor laser driver isconstituted with the SiGe base bipolar transistor as defined above ineither one of the optical receiver system and the optical transmittersystem.

Further, in a millimeter wave transmission system according to thepresent invention having a receiving antenna for millimeter wave(frequency: 30 GHz-300 GHz), a first amplifier for amplifying a receivedelectric signal from the antenna, a receiving mixer for receiving theoutput from the first amplifier and stepping-down the frequency, a firstoscillator, a transmitting mixer for receiving a transmission electricsignal and stepping-up the frequency, a second oscillator, and a secondamplifier for receiving the output of the transmission mixer andamplifying the power,

at least one of the transistors in the first and the second amplifiers,the first and the second oscillators and the transmission mixer isconstituted with the SiGe base bipolar transistor as defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a depth profile of impurity concentrations andGe contents of a bipolar transistor using a SiGe alloy for a base in thefirst embodiment according to the present invention;

FIG. 2 shows an energy band structure in an active region of an Sibipolar transistor in the first embodiment according to the presentinvention;

FIG. 3 shows a cross sectional structure of a bipolar transistor in thefirst to the seventh embodiments according to the present invention;

FIG. 4 shows a cross sectional structure for a main portion of a bipolartransistor in the first to the seventh embodiments according to thepresent invention;

FIG. 5 shows a cross sectional structure for a main portion of mainsteps in a method of manufacturing a bipolar transistor in the first tothe seventh embodiments according to the present invention;

FIG. 6 shows a cross sectional structure for a main portion in mainsteps of a method of manufacturing a bipolar transistor in the first tothe seventh embodiments according to the present invention;

FIG. 7 is a graph showing a depth profile of impurity concentrations andGe contents of a bipolar transistor in the second embodiment accordingto the present invention;

FIG. 8 is a graph showing a depth profile of impurity concentration withand Ge contents of a bipolar transistor in the third embodimentaccording to the present invention;

FIG. 9 is a graph showing a depth profile of impurity concentrations andGe contents of a bipolar transistor in the fourth embodiment accordingto the present invention;

FIG. 10 shows an energy band structure in an active region of a bipolartransistor in the fourth embodiment according to the present invention;

FIG. 11 is a graph showing a depth profile of impurity concentrationsand Ge contents of a bipolar transistor in the fifth embodimentaccording to the present invention;

FIG. 12 is a graph showing a depth profile of impurity concentrationsand Ge contents of a bipolar transistor in the sixth embodimentaccording to the present invention;

FIG. 13 shows an energy band structure in an active region of a bipolartransistor in the sixth embodiment according to the present invention;

FIG. 14 is a graph showing a depth profile of impurity concentrationsand Ge contents of a bipolar transistor in the seventh embodimentaccording to the present invention;

FIG. 15 is a graph showing a relation between a Ge content and a Bdiffusion coefficient in an SiGe alloy layer;

FIG. 16 is a graph comparing the dependency of a highest cut-offfrequency on a collector current density between the transistor in thefirst embodiment according to the present invention and the transistorof the prior art;

FIG. 17 is a graph showing an example of a depth profile of impurityconcentrations and Ge contents in a bipolar transistor using an existenttype selectively epitaxially grown SiGe base;

FIG. 18 shows an energy band structure in an active region of a bipolartransistor using an existent type selectively epitaxially grown SiGebase;

FIG. 19 shows a preamplifier for an optical transmission system in theeighth embodiment according to the present invention;

FIG. 20 is a constitutional view for a front-end module of an opticaltransmission system in the ninth embodiment according to the presentinvention;

FIG. 21 is a constitutional view of an optical transmission system inthe tenth embodiment according to the present invention;

FIG. 22 shows an oscillator in a millimeter wave transmission system inthe eleventh embodiment according to the present invention; and

FIG. 23 shows an oscillator in a millimeter wave transmission system inthe twelfth embodiment according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of a bipolar transistor according to the presentinvention and optical communication systems and millimeter wavetransmission systems to which the bipolar transistor is applied are tobe explained below with respect to preferred embodiments from 1 to 12and related drawings to each of the embodiments.

Embodiment 1

The first embodiment according to the present invention is to beexplained with reference to FIG. 1 and to FIG. 4. FIG. 3 shows a crosssectional structure of a bipolar transistor using an SiGe alloy layerfor a base in the first embodiment according to the present invention.Cross sectional structures of bipolar transistors in other embodimentsaccording to the present invention are basically identical with thoseshown in this figure. In the drawing, are shown a p-type Si substrate 1,an n⁺-type Si layer 2, a low concentration n-type Si layer 3, an SiO₂film 4, an n⁺-type Si layer 5, an SiO₂ film 6, a non-doped polysiliconlayer 7, an Si₃N₄ film 8, a p-type polysilicon layer 9, an SiO₂ film 10,an n-type Si layer 11, a low concentration n-type SiGe film 12, a p-typeSiGe layer 13, a low concentration n-type SiGe layer 14, a non-dopedsingle crystalline Si layer 15, a p-type polycrystal SiGe layer 16, anSiO₂ film 17, an Si₃N₄ film 18, an n⁺-type polycrystal Si film 19, ann⁺-type Si layer 20, SiO₂ films 21, 22, an n-type polycrystal Si film23, and metal electrodes 24-26. Among the metal electrodes, 24 denotesan emitter electrode, 25 denotes a base electrode and 26 denotes acollector electrode.

FIG. 4 is a detailed cross sectional structure for a main portion of theSiGe base transistor shown in FIG. 3. Those reference numerals in thisfigure are identical with those shown in FIG. 1. In this case, then⁺-type Si layer 2 serves as a collector, the p-type polysilicon layer 9serves as a base lead electrode, the n⁺-type polycrystal Si film 19 andthe n⁺-type Si layer 20 serves as an emitter.

FIG. 1 is a graph showing a depth profile of impurity concentrations andGe contents of the SiGe base transistor in the first embodiment shown inFIG. 3 and FIG. 4, at a portion along a broken line A in FIG. 4. Thepeak concentration of B in the base layer is 1×10²⁰ cm⁻³. The Ge contentis made uniform as about 20% in the B-doped region and has asubstantially trapezoidal profile with the maximum value at about 30% inthe emitter-base junction region and in the base-connection junctionregion adjacent therewith.

FIG. 2 shows an energy band structure in an active region of thetransistor in the first embodiment. As can be seen from the figure, anodge is formed in the valence band and at the edge of the base layer onthe side of the collector. On the other hand, no potential barrierhindering the electron transition is not present in the conduction band.

Then, a method of manufacturing an SiGe base transistor in the firstembodiment explained with reference to FIG. 1 to FIG. 4 is to beexplained with reference to FIG. 5 and FIG. 6. The figures show crosssectional structures for main portions in main steps for manufacturingthe transistor. After forming an n⁺-type Si layer 2 as a buried layer, alow concentration n-type Si layer 3, and an SiO₂ film 4 for deviceisolation on a p-type Si substrate 1, an SiO₂ film 6, a non-dopedpolysilicon layer 7, an Si₃N₄ film 8, a p-type polysilicon layer 9 andan SiO₂ film 10 are deposited by way of a usual chemical vapordeposition (CVD) method (FIG. 5(a)). Then, an SiO₂ film 10 and a p-typepolysilicon layer 9 of a portion on the device forming region areremoved by usual photolithography and etching to form a window reachingthe Si₃N₄ film 8. Subsequently, an SiO₂ film 27 is deposited by the CVDmethod and, further, the SiO₂ film 27 for the portion other than theside wall of the window is removed by anisotropic dry etching. Then, anSi₃N₄ film 28 is deposited by the CVD method and, further, the Si₃N₄film 28 and the Si₃N₄ film 8 for the portion other than the side wall ofthe window are removed by anisotropic dry etching (FIG. 5(b)). Then, thenon-doped polysilicon layer 7 exposed to the bottom of the window isremoved by wet etching and, further, the SiO₂ film 6 exposed thereby tothe bottom of the window is removed by wet etching to expose the lowconcentration n-type Si layer 3 (FIG. 5 (c)). Then, a low concentrationn-type SiGe film 12, a p-type SiGe film 13, a low concentration n-typeSiGe film 14 and a non-doped single crystalline Si layer 15 areselectively grown epitaxially on the low concentration n-type Si layer 3at the bottom of the window successively by an ultra-high vacuum CVDmethod. Further, a p-type polycrystal SiGe layer 16 is grown from theexposed portion of the p-type polysilicon layer 9 simultaneously to joinwith the selective epitaxial layer (FIG. 6(d)). Then, after forming ann-type Si layer 11 by ion implanting P and heating, an SiO₂ film 17 andan Si₃N₄ film 18 are deposited by a usual CVD method and, further, theSi₃N₄ layer 18 and the SiO₂ film 17 for the portion other than the sidewall of the window are removed by anisotropic dry etching and wetetching. Then, a p-doped n⁺-type polycrystal Si layer 19 is depositedand heated by the usual CVD method to diffuse P into the non-dopedsingle crystal layer Si 15 to form an n⁺-type Si layer 20 in thenon-doped single crystalline Si layer 15. Further, an n⁺-typepolycrystal Si layer 19 for the portion other than the area just abovethe opening and the periphery thereof was removed selectively by theusual photolithography and etching (FIG. 6(e)). In the succeeding steps,the transistor shown in FIG. 1-FIG. 4 is completed by way of usual stepsof wiring and formation of intermetal layer.

A method of manufacturing bipolar transistors of other embodimentsaccording to the present invention is also identical basically asdescribed above except for making the profiles different between B andGe in selective epitaxial growing.

In this embodiment, diffusion of B from the B-doped layer to a regionadjacent therewith can be suppressed. Therefore, at a B-doped layerwidth of 20 nm and a peak concentration of 1×10²⁰ cm⁻³ immediately afterthe epitaxial growing for instance, the width of the B-doped regionafter emitter annealing is about 28 nm and it can be reduced to about70% relative to the width of about 40 mm in the prior art. As a result,this can provide an effect capable of shortening the base transit timeof electron to about ½.

Further, in this embodiment, since a nodge is formed to a valence bandat the edge of the base layer on the side of the collector, increase inthe effective base width due to the increase of the collector currentless occurs. Therefore, the collector current at which the cut-offfrequency of the transistor becomes maximum is increased by 50%. As aresult, this can provide an effect of improving the maximum cut-offfrequency to about 40%, in addition to the effect of shortening the basetransit time mentioned above. FIG. 16 compares the dependence of themaximum cut-off frequency on the collector current between thetransistor of this embodiment and the transistor of the prior art.

Embodiment 2

The graph shown in FIG. 7 shows a depth profile of impurityconcentrations and Ge contents in an SiGe base transistor in a thirdembodiment according to the present invention at a portion along thebroken line A in FIG. 4. The peak concentration of B in the base layeris 1×10²⁰ cm⁻³. The Ge content increases from about 15% to about 30%from the B-doped region to the base-collector junction region adjacenttherewith. On the other hand, it has a substantially trapezoidal profilewith the maximum value of about 30% in the emitter-base junction regionadjacent with the B-doped region.

According to this embodiment, since the Ge content has a grade in thebase layer, an electric field for accelerating electrons from theemitter to the collector is formed. Therefore, it can provide an effectcapable of shortening the base transit time of electrodes further byabout 20% compared with that of the transistor in the first embodiment.However, since the total amount of Ge contents is increased comparedwith that in the first embodiment, it causes a side-effect of increasingthe probability for the occurrence of crystal defects.

Embodiment 3

The graph of FIG. 8 shows a depth profile of impurity concentrations andGe contents in the SiGe base transistor in the third embodimentaccording to the present invention at a portion along the broken line inFIG. 4. The B peak concentration in the base layer is 1×10²⁰ cm⁻³. TheGe content increases from about 15% to about 20% in the direction fromthe emitter to the collector in the B-doped region. On the other hand,it forms a substantially trapezoidal profile with the maximum value ofabout 30% in the emitter-base junction region and the base-collectorjunction region adjacent with the B-doped region. The grade of the Gecontent on both ends of the B-doped region is greater than the grade inthe B-doped region.

According to this embodiment, since the Ge content has a grade in thebase layer like that the second embodiment, the base transit time ofelectrons can be made identical with that of the transistor in thesecond embodiment. Further, since the total amount of the Ge contents isless than that in the second embodiment, it has an effect capable ofreducing the probability for the occurrence of crystal defects to alevel substantially equal with that in the first embodiment.

Further, in this embodiment, since a nodge is formed in the valence bandat the edge of the base layer on the side of the collector like that inthe first embodiment, increase of the effective width of the base due tothe increase of the collector current less occurs at which the cut-offfrequency of the transistor becomes maximum is increased by 25% and, asa result, it provides an effect of improving the cut-off frequency by8%.

Embodiment 4

The fourth embodiment according to the present invention is to beexplained with reference to FIG. 9 and FIG. 10. FIG. 9 is a graphshowing the depth profile of impurity concentrations and Ge contents inthe SiGe base transistor in the fourth embodiment according to thepresent invention at a portion along the broken line A shown in FIG. 4.The B peak concentration in the base layer is 1×10²⁰ cm⁻³. The Gecontent is uniform as about 20% in the B-doped region and theemitter-base junction region adjacent therewith. On the other hand, ithas a substantially trapezoidal profile with the maximum value of about35% in the base-collector junction region adjacent with the B-dopedregion. FIG. 10 shows an energy band structure in an active region ofthe transistor in the fourth embodiment. As can be seen from the figure,a nodge is formed in the valence band at the edge of the base on theside of the collector. Further, no potential barrier inhibiting theelectron transition are not present in the conduction band of thistransistor.

According to this embodiment, the Ge content is decreased in theemitter-base junction region and, instead, the Ge content is increasedin the base-collector region compared with the transistor in the firstembodiment. Therefore, the B diffusion speed in the base-collectorjunction in which B tends to diffuse more can be lowered withoutincreasing the total amount of the Ge contents. As a result, the widthof the B-doped region can be decreased by about 7%, that is, to 26 mmcompared with the first embodiment. As a result, it can provide aneffect capable of reducing the base transit time of electrons by about14%.

Embodiment 5

The graph of FIG. 11 shows the depth profile of the impurityconcentrations and Ge contents in the SiGe base transistor in the fifthembodiment according to the present invention at a portion along thebroken line A shown in FIG. 4. The B peak concentration is 1×10²⁰ cm⁻³in the base layer. The Ge content increases from about 20% to about 28%in the direction from the emitter to the collector in the B-dopedregion. On the other hand, a region with a uniform content at about 20%is present in the emitter-base junction region adjacent with the B-dopedregion. Further, it has a substantially trapezoidal profile with themaximum value of about 35% in the base-collector junction region. Thegrade of the Ge content at the edge of the B-doped region on the side ofthe collector is greater than the grade in the B-doped region.

According to this embodiment, since the Ge content has a grade in thebase layer, an electric field for accelerating electrons from theemitter to the collector is formed. Accordingly, it can provide aneffect capable of decreasing the base transit time of electron furtherby about 20% compared with the transistor in the fourth embodiment.

Embodiment 6

The sixth embodiment according to the present invention is to beexplained with reference to FIG. 12 and FIG. 13. FIG. 12 is a graphshowing the depth profile of impurity concentrations and Ge contents inthe SiGe base transistor in the sixth embodiment according to thepresent invention at a portion along the broken line A shown in FIG. 4.The B peak concentration in the base layer 1×10²⁰ cm⁻³. The Ge contentis uniform at about 20% in the B-doped region and decreases abruptlyfrom the edge of the B-doped region to the emitter-collector junctionregion adjacent therewith. On the other hand, it has a substantiallytrapezoidal profile with the maximum value of about 40% in thebase-collector junction region adjacent with the B-doped region. FIG. 13shows an energy band structure in an active region of the transistor inthe sixth embodiment. As can be seen from the figure, a nodge is formedin the valence band at the edge of the base on the side of thecollector. Further, no potential barrier inhibiting the electrontransition is present in the conduction band of this transistor.

According to this embodiment, the Ge content in the base-collectorjunction region is increased more instead of decreasing the Ge contentin the emitter-base junction region to 0 compared with the transistor inthe fourth embodiment. Accordingly, the B diffusion speed in thebase-collector junction can be further lowered without increasing thetotal amount of Ge contents. As a result, this can provide an effectcapable of making the width of the B-doped region and the base transittime of electrons substantially equal with those in the fourthembodiment.

Embodiment 7

The graph of FIG. 14 shows the depth profile of the impurityconcentrations and the Ge contents in the SiGe base transistor in theseventh embodiment according to the present invention at a portion alongthe broken line A shown in FIG. 4. The B peak concentration is 1×10²⁰cm⁻³ in the base layer. The Ge content increases from about 0% to about28% in the direction from the emitter to the collector in the B-dopedregion. On the other hand, it has a substantially trapezoidal profilewith the maximum value of about 40% in the base-collector junctionregion. The grade of the Ge content at the edge of the B-doped region onthe side of the collector is larger than the grade in the B-dopedregion.

According to this embodiment, since the Ge content has a grade in thebase layer, an electric field for accelerating electrons from theemitter to the collector is formed. Accordingly, it can provide aneffect capable of shortening the base transit time of electrons byfurther about 20% compared with the transistor in the sixth embodiment.

Embodiment 8

FIG. 19 shows a diagram for a preamplifier circuit in an opticaltransmission system illustrating an eighth embodiment according to thepresent invention. This embodiment is an example of using the SiGe basebipolar transistor in the previous embodiments of a transistor 301 foramplification, and transistors 302 and 303 for a buffer. This is acircuit of amplifying an input from a photodiode 306, and outputting thesame from an output buffer 307 by way of an amplifier comprisingtransistors 301, 302 and 303 and registers 304 and 305. This circuit hasa frequency band higher than 40 GHz by the use of the SiGe base bipolartransistor in the embodiment described above.

The frequency band of this circuit can be made to 40 GHz or higher alsoby using a transistor comprising a compound semiconductor such as GaAsinstead of the SiGe base bipolar transistor. However, this embodimentusing the SiGe base bipolar transistor has an advantageous featurecompared with the case of using the compound semiconductor device inthat the cost is reduced since the material for the substrate isinexpensive, consumes less electric power since the shrink of the devicemanufactured by using the well established Si fabrication processes iseasy and the reliability is high since this is an Si series device.

Embodiment 9

FIG. 20 shows a front-end module including a photodiode and apreamplifier in an optical receiving module showing a ninth embodimentaccording to the present invention. This embodiment is an example ofapplying the preamplifier comprising the SiGe base bipolar transistor ofthe embodiments described previously to a front end module. An opticalsignal inputted from an optical fiber 401 is focused by a lens 402 andconverted by a photodiode IC 403 into an electric signal. The electricsignal is amplified by a preamplifier IC 404 through a wiring 405 on asubstrate 407 and then outputted from an output node 406.

Embodiment 10

FIG. 21 is a constitutional view for an optical transmission systemillustrating a tenth embodiment according to the present invention. Thisembodiment is an example of applying the SiGe base bipolar transistor ofthe previous embodiments to the circuits in both of the transmissionsystems of an optical transmission module 513 for transmitting data at asuper-high speed and an optical receiver module 514 for receiving thedata.

The module comprises a multiplexer 501 for processing electric signals510 to be transmitted, a semiconductor laser 503, a semiconductor laserdriving analog circuit 502 for driving a semiconductor laser 503, apreamplifier 505 for amplifying a received electric signal 512 preparedby conversion of transmitted optical signal 511 by a photodiode 504, aswell as each of analog circuits of an automatic-gain-control amplifier506, a clock extraction circuit 507 and a decision circuit 508, and ademultiplexer 509 as a digital circuit. In this embodiment, transistorsin the multiplexer 501, the analog circuit 502 for the semiconductorlaser driver, the preamplifier 505, the automatic-gain-control amplifier506, the clock extraction circuit 507, the decision circuit 508 and thedemultiplexer 509 are SiGe base bipolar transistors in the previousembodiments. Since the SiGe base bipolar transistor has a cut-offfrequency and a maximum cut-off frequency as high as 200 GHz, thistransmission system can transmit/receive signals at a large capacity of40 G bits or more per one sec at a super-high speed. This embodimentalso has a similar feature as that in the eighth embodiment.

Embodiment 11

FIG. 22 shows an oscillator circuit in a millimeter wave transmissionsystem illustrating an eleventh embodiment according to the presentinvention. This circuit comprises a transistor 601, a varactor diode602, ¼ λ stubs 603 and 604, feedback stubs 607 and 608 and transmissionlines 609 and 610 as a matching circuit. This embodiment is an exampleof using the SiGe base bipolar transistor in the previous embodiments tothe transistor 601. This is a circuit for supplying millimeter wave bandfrequency signals to a transmission/receiver mixer. This circuit has aband frequency of 60 GHz or higher by using the SiGe base bipolartransistor of the embodiments described previously.

The band frequency of this circuit can be made to 60 GHz or higher alsoby using a transistor comprising a compound semiconductor such as GaAsinstead of the SiGe base bipolar transistor. However, this embodimentusing the SiGe base bipolar transistor has an advantageous featurecompared with the case of using the compound semiconductor device inthat the cost is reduced since the material for the substrate isinexpensive, it consumes less electric power since the shrink of thedevice is easy by using the well established Si fabrication processes,and the reliability is high since this is an Si device.

Embodiment 12

FIG. 23 is a constitutional view for a millimeter wave transmissionsystem illustrating a twelfth embodiment according to the presentinvention. This embodiment is an example of applying the SiGe basebipolar transistor in the previous embodiments to the circuit in themillimeter wave transmission system.

This millimeter wave transmission system comprises a transmittingoscillator 701, a transmitting mixer 702 for receiving a transmissionelectric signal and stepping up the frequency, filters 703 and 710, apower amplifier 704 for receiving the output from the transmittingmixer, a transmitter/receiver switch 705, a receiving antenna 706 formillimeter wave, a low noise amplifier 707 for amplifying the receivedelectric signal from the antenna, a receiving oscillator 708, and areceiving mixer 709 for receiving the output from the low noiseamplifier for stepping-down the frequency. In this embodiment,transistors in the transmitting oscillator 701, the transmitting mixer702, the power amplifier 704, the low noise amplifier 707, the receivingoscillator 708, and the receiving mixer 709 are SiGe base bipolartransistors in the previous embodiments. Since the cut-off frequency andthe maximum cut-off frequency are as high as 200 GHz, thistransmitting/receiving system can transmit/receive millimeter wavesignals at a frequency of 60 GHz or higher. This embodiment also has thesame advantageous features as the eighth embodiment.

According to the present invention, the base diffusion speed in theemitter-base junction region and the base-collector junction regionadjacent with the B-doped layer of the base can be lowered. As a result,at the width of the B-doped layer for the base of 20 nm and the peakconcentration of 1×10²⁰ cm⁻³ immediately after epitaxial growing, thewidth of the B-doped region after emitter annealing is about 26 nm to 28nm, which can be reduced by about 65% to 70% relative to about 40 mmwidth in the prior art. As a result, base transit time of electrons canbe decreased to about 42%-50%.

Further, according to the present invention, since a nodge is formed inthe balance electron band at the edge of the base on the side of thecollector, increase of the effective base width due to the increase ofthe collector current occurs less. Accordingly, the collector current atwhich the cut-off frequency of the transistor reaches maximum can beincreased by about 50% compared with the prior art. As a result, it canprovide an effect of improving the maximum cut-off frequency by 40% to70%, coupled with the effect of shortening the base transit time.

What is claimed is:
 1. A bipolar transistor using a B-doped Si and Gealloy for a base in which the maximum value of a Ge content in anemitter-base junction depletion region and a base-collector junctiondepletion region is greater than an average value in a base layer,wherein said Ge content increases abruptly from a vicinity of an edge ofthe base layer on the emitter side to the emitter, the edge of the baselayer on the emitter side being in the depth of 70-80 nm.
 2. A bipolartransistor according to claim 1, wherein a grade of said Ge content in afirst region in which the grade of Ge content increases from a vicinityof an edge of a base layer on a collector side to the collector isgreater than the grade of said Ge content in a second region disposed inthe base layer and adjacent to said first region.
 3. A bipolartransistor according to claim 1, wherein the maximum of a B content ofsaid B-doped Si and Ge alloy is 1×10²⁰ cm⁻³.
 4. A bipolar transistorusing a B-doped Si and Ge alloy for a base in which for maximum value ofa Ge content in an emitter-base junction depletion region and abase-collector junction depletion region is greater than an averagevalue in a base layer, wherein said Ge content increases abruptly from avicinity of an edge of the base layer on the emitter side to the emitterin a region where B content decreases abruptly from a vicinity of anedge of the base layer on the emitter side to the emitter, the edge ofthe base layer on the emitter side being in the depth of 70-80 nm.
 5. Abipolar transistor according to claim 4, wherein a grade of said Gecontent in a first region in which the grade of Ge content increasesfrom a vicinity of an edge of a base layer on a collector side to thecollector is greater than the grade of said Ge content in a secondregion disposed in the base layer and adjacent to said first region. 6.A bipolar transistor according to claim 4, wherein the maximum of a Bcontent of said B-doped Si and Ge alloy is 1×10²⁰ cm⁻³.
 7. A bipolartransistor comprising: a substrate; a first Si layer formed on saidsubstrate; a second Si layer formed on said first Si substrate, saidsecond Si layer has a concentration lower than said first Si layer; afirst SiGe film formed on said second Si layer; a second SiGe filmformed on said first SiGe film, said second SiGe film has an electricconductivity opposite to said first SiGe film; a third SiGe film formedon said second SiGe film, said third SiGe film has an electricconductivity opposite to said second SiGe film; a third Si layer formedon said third SiGe film, said third Si layer has a concentration higherthan said second Si layer; wherein said first and second Si layers arecomprised with a collector and said second SiGe film is comprised with abase and said third Si layer is comprised with an emitter, wherein saidbipolar transistor uses a B-doped Si and Ge alloy for said base in whichthe maximum value of a Ge content in an emitter-base junction depletionregion and a base-collector junction depletion region is greater than anaverage value in said base layer, and wherein said Ge content increasesabruptly from a vicinity of an edge of said base layer on the emitterside to said emitter.
 8. A bipolar transistor according to claim 7,wherein said substrate is p-type Si substrate.
 9. A bipolar transistoraccording to claim 7, wherein a grade of said Ge content in a firstregion in which the grade of Ge content increases from a vicinity of anedge of a base layer on a collector side to the collector is greaterthan the grade of said Ge content in a second region disposed in thebase layer and adjacent to said first region.
 10. A bipolar transistoraccording to claim 7, wherein the maximum of a B content of said B-dopedSi and Ge alloy is 1×10²⁰ cm⁻³.